1. Field of the Invention
The present invention relates to a ratchet-type tensioner for applying an appropriate tension to a timing chain of a vehicle engine or to a like device. More particularly, the invention relates to a mechanism for eliminating backlash of a plunger used in a ratchet-type tensioner.
2. Description of the Related Art
Conventionally, a ratchet-type tensioner as shown in FIG. 3 is used with a chain drive for transmitting rotational motion from the crankshaft to the cam shaft of an engine. The tensioner is adapted to apply an appropriate tension to the slack side of a timing belt or timing chain to thereby suppress vibration which would otherwise occur on the timing belt or timing chain when the belt or chain travels.
In FIG. 3, a ratchet-type tensioner A1 is mounted on an engine body on the slack side of a chain A6 (timing chain). The chain A6 is fitted around and extends between a driving sprocket A3 rotated by a crankshaft A2 of an engine and a driven sprocket A5 fixed on a cam shaft A4.
In the ratchet-type tensioner A1, a plunger A8 projects from the front face of a tensioner housing A7 such that it can undergo reciprocating motion. A tip F of the plunger 8 presses the rear side of a tension lever A10 at a position near the free end of the tension lever A10. The tension lever A10 is pivotally supported on an engine body by means of a support shaft A9. Through such arrangement, a shoe surface A11 of the tension lever A10 is brought into slidable contact with the slack side of the chain A6 to thereby apply tension to the chain A6.
The tensioner housing A7 has a plunger accommodation bore A12 formed therein. The plunger A8 is fitted into the bore A12 such that it may undergo reciprocating motion.
The plunger A8 has a hollow portion A13 formed therein. The hollow portion A13 opens to the end surface of the plunger A8 which faces the bottom of the plunger accommodation bore A12. A plunger spring A14 is inserted into the hollow portion A13 and disposed between the bottom of the plunger accommodation bore A12 and that of the hollow portion A13, thereby urging the plunger A8 at all times in such a direction that the plunger A8 is projected from the tensioner housing A7.
A ratchet A16 is pivotally supported on the tensioner housing A7 by means of a ratchet pin A15. As shown in FIG. 4, the ratchet A16 has ratchet pawls A17 and A18 which mesh with rack teeth T formed on an outer surface of the plunger A8.
A ratchet spring A19, which is disposed between the tensioner housing A7 and the ratchet A16, urges the ratchet A16 to rotate about the ratchet pin A15 so that the ratchet pawls A17 and A18 are in engagement with the rack teeth T at all times. Through engagement between the ratchet pawls A17 and A18 and the rack teeth T, the plunger A8 is prevented from moving in a receding direction (to the left in FIG. 3).
In the tensioner A1, when the tension of the chain A6 of FIG. 3 is high, the plunger A8 held at a position depicted with a solid line in FIG. 4 gradually projects beyond the position depicted with the solid line of FIG. 4 as the chain A6 elongates. Following the projection of the plunger A8, the ratchet A16 rotates counterclockwise about the ratchet pin A15 as shown in FIG. 4.
At this time, the fronts-side ratchet pawl A18, which has meshed with a rack tooth T, leaves the rack tooth T, and only the rears-side ratchet pawl A17 is in contact with the rack tooth T. Subsequently, when the plunger A8 projects further until the tip F advances over a distance d, the ratchet pawl A17 disengages the rack tooth T. As a result, the ratchet A16 rotates clockwise due to a force applied to the ratchet A16 by the ratchet spring A19.
Subsequently, when, as a result of the tension of the chain A6 increasing again due to an increase in the distance between the axes of the sprockets A3 and A5 caused by thermal expansion, the tension lever A10 presses the tip F of the plunger A8 located at a position depicted with broken lines in FIGS. 4 and 5, i.e., at a position reached through projection over the distance d in FIG. 4, the plunger A8 can slightly recede in an amount of a backlash b FIG. 5, or over the distance between the tip F depicted with the broken line and the tip F depicted with a solid line in FIG. 5.
A path C of movement of the tip of the ratchet pawl A17 interferes with a rack tooth T depicted with the solid line in FIG. 5. Thus, in order for the plunger A8 to move from the position depicted with the solid line in FIG. 4 to the position depicted with the solid line in FIG. 5, i.e., to advance in an amount corresponding to the pitch of the rack teeth T, the plunger A8 must advance the distance d, which is longer than the rack tooth pitch. Otherwise, the ratchet pawl A17 cannot pass the interfering range. Thus, the backlash b equals the difference between the distance d and the array pitch of the rack teeth T.
The presence of the backlash b prevents howling which would otherwise occur during chain operation due to an excessive tension of the chain A6, which is produced when the distance between the axis of the crankshaft A2 and the axis of the cam shaft A4 increases due to thermal expansion. Notably, Japanese Patent Publication No. 3-8415 discloses a ratchet-type tensioner having an intentionally increased backlash.
However, with a recent technological innovation in chains, silent chains tend to be used as timing chains for vehicle use. Thus, even when a chain tension becomes excessively large, a silent chain hardly generates howling. By contrast, at present, noise derived from vibration of a running chain is rather a problem. Noise derived from chain vibration tends to increase with plunger backlash.
To cope with the noise problem, in many ratchet-type tensioners, oil is introduced via a check valve into an internal space defined by a tensioner housing and a plunger. When an impact force acts on the plunger from a chain, the impact force is hydraulically damped to thereby suppress chain vibration. However, incorporation of the check valve into the tensioner housing causes an increase in tensioner size as well as structural complication, resulting in an increase in manufacturing cost.